US3432367A - Process for the reduction of hot-rolled strip with very fine structural grain size - Google Patents

Description

United States Patent 3,432,367 PROCESS FOR THE REDUCTION OF HOT-ROLLED giIZREIIP WITH VERY FINE STRUCTURAL GRAIN Wilhelm Stich, Wattenscheid-Hontrop, and Adolf Sickbert, Wattenscheid-Eppendorf, Germany, assignors to Heppenstall Company, Pittsburgh, Pa.

No Drawing. Filed Apr. 5, 1965, Ser. No. US. Cl. 148-12 Int. Cl. C21d 7/13 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a process for the reduction of hot-rolled strip to produce a stable fine grained steel and particularly a fine grained steel which will not coarsen on aging.

The problem of producing a stable fine grained steel is an old and well recognized problem. It has long been recognized that hot-rolling would reduce grain size but that the fine grain so produced was not stable and would grow on aging. In order to overcome this problem, it has been the practice to add alloying ingredients which would induce fine grain formation such as aluminum, vanadium, titanium and zirconium. Such materials are, of course, expensive, require additional handling and tend to promote segregation of impurities in the metal.

More recently, it has been proposed in application Ser. No. 445,405, and now abandoned, owned by the owner of the present application, to treat hot-rolled strip with Water under pressure during hot-rolling prior to One of the last three reductions of the finishing mill; whereby the strip or band is quickly cooled and descaled by the water under pressure. The cooling of the band, according to this process, should on the average amount to about 50 to 100 C. This method has a number of favorable effects. It produces a fine grain which is resistant to growth, and it produces a smooth surface.

The present invention provides a method for producing fine grained steels which eliminates the subsequent problem of grain growth and which produces the same smooth surface as disclosed in application Ser. No. 445,405. This is accomplished with the use of a small amount of cooling media. The present process provides the steps of casting a coarse grain vacuum treated steel free from elements which lead to fine grain formation, solidifying said steel, rapidly reducing the cross-section of the solidified steel at an elevated temperature and at a pressure sufficient to reduce the coarse grain structure to a desired fine grain structure and immediately cooling the reduced steel below the grain growth temperature prior to the inception of any grain growth.

The structure, which results from the treatment of this invention, is very fine grained; and the fine grain quality remains substantially unchanged even after subsequent annealing treatments which would have caused considerable grain growth in the materials of the prior art. It is, for example, possible to obtain a fine grain quality in hotrolled band of deep drawing quality with a final thickness of between three to eight minutes and an ASTM grain size of six to eight.

An unalloyed steel with a carbon content of 0.75% was treated according to the process outlined above, i.e., vacuum treated and cast to a coarse grained structure, hotrolled and sprayed with water in the last stand to produce an AST M grain size range of seven to nine. This size remained unchanged after an anneal of one hour at 680 C. in air.

The surface of the hot-rolled band or strip is so smooth that it may be considered mirror-smooth for practical purposes. This is attributed to the fact that the surface roughness previously consisting of varying scale particles can be completely smoothed out by the rolling operation of this invention.

We have found that an original coarse grain or alternatively a large grain growth possibility, is necessary if a fine grain structure is to be obtained in the final rolled sheet by hot-rolling as described herein and as distinguished from those methods of producing fine grained steels by metallurgical additions in the melt.

The coarse grain in the slab or ingot must also be induced as a result of vacuum treatment by means of any of the well-known vacuum methods such as pouring stream degassing. This resulting vacuum induced coarse grain must be reduced under the high roll pressures of modern strip mills and finally quickly cooled. If the final quick cooling step is not taken, the final strip would possibly grow to a coarse grain even coarser than the starting grain size. For this reason, it is essential that the strip be sprayed with coolant while in the rolling mill. The resulting eifect of the spraying with coolant is greater, the nearer it occurs to the end of the rolling process. It is, therefore, essential in this invention that the steel, which as a result of the vacuum treatment has been induced to form a coarse grain, be reduced by high rolling pressures during the rolling of the strip and treated with cooling water under pressure during the rolling of the strip and prior to the inception of any grain growth. It is, therefore, particularly effective if the steel band receives the impact of the high pressure cooling water in at least one of the last three reductions in the rolling mill.

In the practice of this invention, the addition of those elements, i.e., aluminum, vanadium, titanium, and zirconium, which are usually added to induce a fine grain condition in steel, should be avoided or limited to amounts which are not sutficient to affect the grain growth of the steel following the vacuum treatment.

While we haveillustrated and described certain pre ferred practices of our invention in the foregoing specification, it will be understood that this invention may be otherwise practiced within the scope of the following claims.

We claim:

1. Process for the production of stable fine grained steel comprising the steps of casting a coarse grain vacuum treated steel free from elements which lead to fine grain formation, solidifying said steel, rapidly reducing the cross-section of the solidified steel at an elevated temperature and at a pressure sufficient to reduce the coarse grain structure to a desired fine grain structure, and immediately cooling the reduced steel below the grain growth temperature prior to the inception of any grain growth.

2. Process for the production of hot-rolled steel band of stable very fine grain characteristics in the finished steel comprising the steps of casting a coarse grain vacuum treated steel free from additions of elements which lead to fine grain formation, solidifying said steel, hot-rolling the solidified steel at normal hot-rolling temperatures in a continuous strip mill having in excess of three reductions at a pressure sufficient to reduce the coarse grain structure to a desired fine grain structure, and immediately cooling the reduced steel by spraying with coolant in any of the last three reductions of said continuous strip mill.

3. A process as claimed in claim 2 wherein the coolant is high pressure water.

4 References Cited UNITED STATES PATENTS 3,201,288 8/1965 Grange 148-12.4 3,250,648 5/1966 Grange et a1. 14812.4 3,245,844 4/1966 Weber 148 FOREIGN PATENTS 538,783 3/1957 Canada. 773,071 4/ 1957 Great Britain.

10 L. DEWAYNE RUTLEDGE, Primary Examiner.

W. W. STALLARD, Assistant Examiner.

U.S. C1. X.R.